
let VSHADER_SOURCE = 
	'attribute vec4  a_Position;\n' + 
	'void main() {\n' + 
	'	gl_Position = a_Position;\n' + 
	'   gl_PointSize = 10.0;\n' + 
	'}';


let FSHADER_SOURCE = 
	'precision mediump float;\n' + 
	'uniform vec4 u_FragColor;\n' + 
	'void main() {\n' + 
	'	gl_FragColor = u_FragColor;\n' + 
	'}\n';


function main() {
	let canvas = document.getElementById('webgl');

	let gl  = getWebGLContext(canvas);

	if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
		console.log("Failed to initialize shaders");
		return;
	}

	let a_Position = gl.getAttribLocation(gl.program, 'a_Position');
	
	if (a_Position < 0) {
		console.log("Failed to get the storage location of a_Position");
		return;
	}

	let u_FragColor = gl.getUniformLocation(gl.program, "u_FragColor");

	canvas.onmousedown = function(ev) {
		click(ev, gl, canvas, a_Position, u_FragColor);
	};

	gl.clearColor(0.0, 0.0, 0.0, 1.0);
	gl.clear(gl.COLOR_BUFFER_BIT);
}

let g_points = [];  //鼠标点击位置数组
let g_colors = []; 

function click(ev, gl, canvas, a_Position, u_FragColor) {
  
  	let x = ev.clientX; // x coordinate of a mouse pointer
  	let y = ev.clientY; // y coordinate of a mouse pointer
  	let rect = ev.target.getBoundingClientRect() ;

  	x = ((x - rect.left) - canvas.width/2)/(canvas.width/2);
  	y = (canvas.height/2 - (y - rect.top))/(canvas.height/2);
  	// Store the coordinates to g_points array
  	g_points.push([x, y ]);

  	if (x >= 0.0 && y >= 0.0) {
  		g_colors.push([1.0, 0.0, 0.0, 1.0]);
  	} else if(x < 0.0 && y < 0.0) {
  		g_colors.push([0.0, 1.0, 0.0, 1.0]);
  	} else {
  		g_colors.push([1.0, 1.0, 1.0, 1.0]);
  	}

  	 // Clear <canvas>
  	gl.clear(gl.COLOR_BUFFER_BIT);

  	let len = g_points.length;
  	for(let i = 0; i < len; i ++) {
  		let xy = g_points[i];
  		let rgba = g_colors[i];
    	// Pass the position of a point to a_Position variable
    	gl.vertexAttrib3f(a_Position, xy[0], xy[1], 0.0);

    	gl.uniform4f(u_FragColor, rgba[0], rgba[1], rgba[2], rgba[3]);

    	// Draw
   		gl.drawArrays(gl.POINTS, 0, 1);
   	}
  }